Data security on the internet is under threat: in the future, quantum computers could decode
even encrypted files sent over the internet in no time. Researchers worldwide are, therefore, experimenting with quantum networks that will enable a paradigm shift in the future when globally connected to form the quantum internet. Such systems would be able to guarantee tap-proof communication through quantum mechanical phenomena such as superposition and entanglement, as well as cryptographic quantum protocols. However, the quantum internet is still in its infancy: high costs coupled with high energy consumption and a high level of complexity for the necessary technologies have prevented quantum networks from scaling easily.
Two researchers at the Institute of Photonics at the Leibniz University Hannover want to remedy this situation. Using frequency-bin coding, they have developed a novel method for entanglement-based quantum key distribution. This quantum mechanical encryption technique uses different light frequencies, i.e. colours, to encode the respective quantum states. The method increases security and resource efficiency. 'Our approach could enable quantum networks to be scaled up in the future while using fewer resources to connect higher numbers of users over greater distances,' says Prof. Dr. Michael Kues, head of the Institute of Photonics and a member of the board of the PhoenixD Cluster of Excellence at Leibniz Universität Hannover. Research into optical technologies and photonic quantum bits is one of the university's key research areas.
Implementing entanglement-based quantum key distribution using frequency as a degree of freedom has two advantages. 'Firstly, compared to polarisation, frequency is very robust against noise, which environmental factors such as temperature fluctuations and mechanical vibrations in the optical fibres used induce and disturb the key transmission,' says Anahita Khodadad Kashi, a doctoral student at the Institute of Photonics. 'The second advantage is that by using the frequency, we were able to reduce the complexity of the process and thus also the costs,' says Khodadad Kashi.
The researchers have succeeded in measuring the quantum states of the light particles using only one detector instead of four highly sensitive photon detectors. To carry out the four measurements required, they used a method called frequency-to-time transfer, which maps frequency components into the photon’s arrival time at the detector. Kues says that this reduced the costs for the standard telecommunications components from around 100,000 Euro to a quarter of that amount. 'In addition, the vulnerability to detector attacks diminishes, and the system becomes more secure,' says Khodadad Kashi.
The method uses not just one but several channels simultaneously. This so-called adaptive frequency division multiplexing also increases the key distribution rate without the need for additional technical devices. 'With this approach, the performance of the quantum network adapts itself dynamically to the current load,' says Khodadad Kashi. 'In the future, our approach will enable dynamic, resource-minimized quantum key distribution between multiple users. This could make quantum networks scalable,' says Kues.' Quantum networks will be an important building block for making critical IT infrastructure more secure, for example, in the banking and healthcare sectors.'
Kues sees a need for further research into the interaction of nanophotonics with quantum optics in order to develop additional methods and components for generating a wide range of quantum states for the multidimensional coding of quantum information. 'With the development of quantum networks, we will experience a new quality of connectivity, capacity, range and security of quantum communication in the future,' says Kues.
The research was funded by TÜV Nord / Alter Technology, the Federal Ministry of Education and Research (BMBF), and the European Research Council (ERC). The research results were published in the journal Light: Science & Applications.
Original article:
Anahita Khodadad Kashi and Michael Kues
Frequency-bin-encoded Entanglement-based Quantum Key Distribution
in a Reconfigurable Frequency-multiplexed Network
Light: Science & Applications (2025)
https://doi.org/10.1038/s41377-024-01696-8
Note for editors:
For further information, please contact Prof. Dr. Michael Kues
(telephone +49 511 762 3539, email: michael.kues@iop.uni-hannover.de) and visit www.iop.uni-hannover.de and www.phoenixd.uni-hannover.de.
